1. Field of the Invention
The present invention relates to a tone recording method which uses at least two dot densities per color.
2. Description of the Prior Art
In a recording method of a tone image by binary recording, that is, by printing or not printing dots, a dither method and a density pattern method have been used. The number of dots to be printed in a unit matrix is controlled by input density data to express a tone. In this method, when a multi-tone image is to be reproduced, the size of the dot matrix must be increased and the resolution is lowered or a texture of a matrix pattern becomes an obstacle, and hence image quality is degraded.
In the past, it has been proposed to reproduce the tone by using at least two, i.e., high and low, dot densities per color. For example, the tone is reproduced by changing the number of high- and low-density dots in the dot matrix. Another tone recording method has also been proposed, in which the number of dots in the dots matrix and the diameter of the dot are changed. In those methods, however, since the minimum dot diameter which can be printed by an ink jet printer is restricted, the change of density in a highlight image area (low density area), particularly in an area from which recording of dots starts on a recording sheet, is large and hence the tone characteristic in that area is degraded. Even if the tonality of an original document smoothly changes, the data actually read includes a swing due to variations in input conditions including the signal from an input sensor. Accordingly, the dots are printed or not printed in the highlight area so that an uneven image is reproduced.
In the past, in order to improve the tone characteristic, particularly in the highlight area, it has been proposed to use inks having different concentrations of the same color. For example, when a high concentration ink and a low concentration ink are used, the low concentration ink should use a very low concentration dye in order to get a high tonality to the highlight area. As a result, a false outline appears at the junction of the high concentration ink and the low concentration ink due to differences between tonalities of the inks. This degrades the image quality. When high, medium and low concentration inks are used to print a low image density area by the low concentration ink, a medium image density area by the medium concentration ink and a high image density area by the high concentration ink, the tonality is improved because the number of tones is larger, but a false outline may still appear due to a difference of the hue. In order to prevent it, it is necessary to unify the hue over the areas of different densities.
It is desired to combine the high, medium and low density ink dots in order to increase the number of tones which can be reproduced. However, when many inks are printed at the same point, the record sheet cannot absorb the inks and the inks smear or the record sheet can become wavy. In an ink jet recorder, since a plurality of heads are spaced from each other, a distance from the head to the record sheet is not uniform and the tone recording is not precisely effected, and in the worst case, the head contacts to the record sheet and the record sheet may be torn.